Data transmitter

ABSTRACT

The data transmission device  1   a  of the present invention includes a driver  10  for sending data, a receiver  20  for receiving the data sent from the driver  10 , a transmission line path  30  for connecting between the driver  10  and the receiver  20,  and a variable impedance element  40  having a controllably variable impedance. The variable impedance element  40  is connected to the transmission line path  30.  The data transmission line device  1   a  can reduce power consumption and occurrence of skew.

TECHNICAL FIELD

The present invention relates to a data transmission device fortransmitting data from a driver to a receiver via a transmission linepath.

BACKGROUND ART

FIG. 11 shows a configuration of a conventional data transmission device200. The data transmission device 200 includes a driver 210 for sendingdata, a receiver 220 for receiving the data sent from the driver 210,and a transmission line path 230 for connecting between the driver 210and the receiver 220. Data is transmitted via the transmission line path230 from the driver 210 to the receiver 220.

The driver 210 includes an output buffer 212 for outputting data ontothe transmission line path 230. The output buffer 212 is connected via apad 214 to the transmission line path 230.

The receiver 220 includes an input buffer 222 for receiving data fromthe transmission line path 230. One input terminal of the input buffer222 is connected via a pad 224 and a stub resistor 232 to thetransmission line path 230.

An end of a terminator resistor 240 is connected to an end on thereceiver 220 side of the transmission line path 230. The other end ofthe terminator resistor 240 is connected to a terminator potentialV_(term).

The amplitude of a data signal on the transmission line path 230 isdetermined by the resistance of the terminator resistor 240 and theoutput impedance of the driver 210. Therefore, with an appropriatesetting of the resistance of the terminator resistor 240 and the outputimpedance of the driver 210, the amplitude of the data signal on thetransmission line path 230 can be limited to a sufficiently small value.

The resistance of the terminator resistor 240 is typically set so as tobe substantially equal to the characteristic impedance Z of thetransmission line path 230. This prevents data sent from the driver 210from being reflected at the end on the receiver 220 side of thetransmission line path 230.

However, the use of the terminator resistor 240 for terminating thetransmission line path 230 causes a problem such that there is powerconsumption in the absence of data transmission on the transmission linepath 230. This is because when data is held at a HIGH level, a directcurrent (I_(sink)) flows from the terminator potential V_(term) to thedriver 210 via the terminator resistor 240; and when data is held at aLOW level, a direct current (I_(source)) flows from the driver 210 tothe terminator potential V_(term) via the terminator resistor 240.

Also, in the presence of data transmission, since a direct current flowsvia the terminator resistor 240, the slopes of a waveform showing thetransition of the potential of the transmission line path 230 becomesmild as the potential difference between the potential of thetransmission line path 230 and the terminal potential V_(term) isincreased (see FIG. 12). This often causes skew.

Further, the output impedance of the driver 210 when the driver 210outputs data of the HIGH level is not always in agreement with theoutput impedance of the driver 210 when the driver 210 outputs data ofthe LOW level. When these are not in agreement with each other, theabsolute value of the direct current (I_(source)) flowing from thedriver 210 to the terminal potential V_(term) is not identical to theabsolute value of the direct current (I_(sink)) flowing from theterminal potential V_(term) to the driver 210. Therefore, the value ofthe potential amplitude of the transmission line path 230 from theterminal potential V_(term) when the driver 210 outputs the HIGH leveldata is different from the value of the potential amplitude of thetransmission line path 230 from the terminal potential V_(term) when thedriver 210 outputs the LOW level data.

This means that the terminal potential V_(term) is shifted from a middlevalue between a potential (Hi-potential) corresponding to the HIGH leveldata and a potential (Lo-potential) corresponding to the LOW level data.For instance, in an example shown in FIG. 12, the terminal potentialV_(term) is 1.1 V; the Hi-potential is 1.5 V; and the Lo-potential is0.8 V.

The receiver 220 determines whether data on the transmission line path230 has the HIGH level or the LOW level using the terminal potentialV_(term) as a reference potential. Therefore, when the terminalpotential V_(term) is shifted from the middle value of the Hi-potentialand the Lo-potential, the time which it takes data to transit from theLOW level to the HIGH level is different from the time which it takesdata to transit from the HIGH level to the LOW level. This isresponsible for skew occurring when the receiver 220 latches data on thetransmission line path 230 in synchronization with a predetermined clocksignal.

An object of the present invention is to provide a data transmissiondevice in which power consumption is reduced.

Another object of the present invention is to provide a datatransmission device in which occurrence of skew is prevented.

DISCLOSURE OF THE INVENTION

A data transmission device according to the present invention includes adriver for sending data; a receiver for receiving data sent from thedriver; a transmission line path for connecting between the driver andthe receiver; and a variable impedance element having a controllablyvariable impedance. The variable impedance element is connected to thetransmission line path.

According to this invention, by controlling the impedance value of thevariable impedance element, a reduction in power consumption andprevention of skew occurrence can be optimized.

For example, when the data transmission device is operated at a lowspeed, skew is unlikely to occur. Therefore, in this case, the impedancevalue of the variable impedance element is controlled in such a manneras to decrease the impedance value of the variable impedance element.This prevents a direct current from flowing through the transmissionline path. As a result, power consumed by the data transmission devicecan be reduced. When the data transmission device is operated at a highspeed, skew is likely to occur. Therefore, in this case, the impedancevalue of the variable impedance element is controlled in such a manneras to agree with the impedance of the transmission line path. Thisprevents data from being reflected at an end of the transmission linepath. As a result, occurrence of skew is prevented.

The impedance value of the variable impedance element may be changedaccording to a potential of the transmission line path.

For example, when the potential difference between the potential of thetransmission line path and the terminal potential is less than apredetermined value, the impedance value of the variable impedanceelement may be controlled in such a manner as to increase the impedancevalue of the variable impedance element. This allows data to transitfrom a LOW-level to a HIGH-level (or the HIGH-level to the LOW-level) ata high speed. Further, when the potential difference between thepotential of the transmission line path and the terminal potential isgreater than a predetermined value, the impedance value of the variableimpedance element may be controlled in such a manner as to decrease theimpedance value of the variable impedance element. This restricts theamplitude of data and prevents data reflection.

The impedance value of the variable impedance element may be changedaccording to a control signal input from the outside of the variableimpedance element.

For example, when data is transmitted data high speed, a control signalwhich demands that the impedance value of the variable impedance elementis set to a low value is input to the variable impedance element. Thevariable impedance element decreases the impedance in response to thecontrol signal. This prevents data from being reflected at an end of thetransmission line path. As a result, occurrence of skew is prevented.Further, when data transmission is on standby or data is transmitted ata low speed, a control signal which demands that the impedance value ofthe variable impedance element is set to a high value is input to thevariable impedance element. The variable impedance element increases theimpedance in response to the control signal. This prevents a directcurrent from flowing through the transmission line path. As a result,power consumed by the data transmission device can be reduced.

The impedance value of the variable impedance element and an outputimpedance of the driver may be changed in association with each other.In particular, the output impedance of the driver may be changedaccording to the impedance value of the variable impedance element.

For example, when data transmission is on standby or data is transmittedat a low speed, the impedance value of the variable impedance element isset to a high value. The output impedance of the driver is set to a highvalue in response to that the impedance value of the variable impedanceelement has been set to a high value. This makes it possible that thelevel of a Hi-potential corresponding to the HIGH-level data and thelevel of a Lo-potential corresponding to the LOW-level data aresubstantially equal to values which are obtained when the impedancevalue of the variable impedance element is set to the low value. Thismakes it easy to determine whether transmitted data is at the HIGH levelor at the LOW level.

The variable impedance element may include a first diode and a seconddiode connected in parallel. A direction of a current flowing throughthe first diode is opposite to a direction of a current flowing throughthe second diode.

This variable impedance element has an extremely high impedance valueuntil either of the first or second diode is biased in the forwarddirection. This variable impedance element has an extremely lowimpedance value when either of the first or second diode is biased inthe forward direction.

Since the potential of the transmission line path is clamped with thefirst and second diodes, the potential of the transmission line pathtransits between a potential (V_(term)+V_(f)) and a potential(V_(term)−V_(f)) where V_(term) is the terminal potential and is at themiddle of the two potentials; and V_(f) is the forward direction voltageof the first and second diodes. For this reason, a time in which datatransits from the LOW level to the HIGH level becomes substantiallyequal to a time in which data transits from the HIGH level to the LOWlevel. As a result, occurrence of skew is unlikely to occur.

Further, the impedance value of the variable impedance element is set toa high value during the time period of the data transition. For thisreason, a drive load which is applied to the driver during the timeperiod of the data transmission is only the capacitance of thetransmission line path. Therefore, data transits at a constant highspeed. This plays a role in prevention of skew occurrence.

The variable impedance element may further include a resistor connectedin series to the first and second diodes connected in parallel.

Adjustment of the resistance of the resistor can adjust the impedancewhen the first or second diode is biased in the forward direction.

A resistance of the resistor may be substantially equal to acharacteristic impedance of the transmission line path; and a forwarddirection voltage of the first and second diodes may be substantiallyequal to an amplitude of a potential of the transmission line path froma predetermined terminal voltage, the amplitude being generated when thedriver outputs the data onto the transmission line path.

Thus, by setting the resistance of the resistor and the forwarddirection voltage of the first and second diodes, the impedance value ofthe variable impedance element in a state such that either the first orsecond diode is biased in the forward direction is substantially equalto the characteristic impedance of the transmission line path. This canprevent data reflection effectively. Further, even when either of thefirst or second diode is biased in the forward direction, the amplitudeof the potential of the transmission line path from the terminalpotential is substantially in agreement with the forward directionvoltage of the first and second diodes. For this reason, the time inwhich data transits from the LOW level to the HIGH level and the time inwhich data transits from the HIGH level to the LOW level becomesubstantially equal to each other. As a result, skew is unlikely tooccur.

Another data transmission device according to the present inventionincludes a driver for sending data; a receiver for receiving data sentfrom the driver; first and second transmission line paths for connectingbetween the driver and the receiver; a first variable impedance elementhaving a first controllably variable impedance; and a second variableimpedance element having a second controllably variable impedance. Thefirst variable impedance element is connected to the first transmissionline path, and the second variable impedance element is connected to thesecond transmission line path.

According to this invention, by controlling the impedance value of thefirst variable impedance element and the impedance value of the secondvariable impedance element, a reduction in power consumption andprevention of skew occurrence can be optimized.

The first variable impedance element may include first and seconddiodes; the anode of the first diode may be connected to a predeterminedfirst potential; the cathode of the first diode may be connected to thefirst transmission line path; the anode of the second diode may beconnected to the first transmission line path; and the cathode of thesecond diode may be connected to a predetermined second potential lowerthan the predetermined first potential: the sum of the forward directionvoltages of the first and second diodes may be greater than a potentialdifference between the predetermined first potential and thepredetermined second potential; the second variable impedance elementincludes third and fourth diodes; the anode of the third diode may beconnected to a predetermined third potential; the cathode of the thirddiode may be connected to the second transmission line path; the anodeof the fourth diode may be connected to the second transmission linepath; and the cathode of the fourth diode may be connected to apredetermined fourth potential lower than the predetermined thirdpotential; and the sum of the forward direction voltages of the thirdand fourth diodes may be greater than a potential difference between thepredetermined third potential and the predetermined fourth potential.

With the first variable impedance element so constructed, when thepotential of the transmission line path is between the potential(V_(term1)−V_(f)) and the potential (V_(ss)+V_(f)), the transmissionline path is connected to the potential V_(term1) or the potentialV_(SS) via the element having an extremely high impedance. Here,V_(term1) denotes the first potential, V_(SS) denotes the secondpotential, and V_(f) denotes the forward voltage of the first and secondvoltages. For this reason, data transits at a high speed.

Further, when the potential of the transmission line path becomes lessthan the potential (V_(term1)−V_(f)) or greater than (V_(ss)+V_(f)), thefirst or second diode is biased in the forward direction, whereby thetransmission line path is connected to the potential V_(term1) or thepotential V_(SS) via the element having an extremely low impedance. Forthis reason, the level of a Hi-potential corresponding to the HIGH-leveldata and the level of a Lo-potential corresponding to the LOW-level dataare clamped around the potential (V_(term1)−V_(f)) or the potential(V_(ss)+V_(f)). This restricts the amplitude of data.

The same applies to the second variable impedance element.

Thus, data transit at a high speed and the amplitude of data isrestricted. As a result, it is possible to obtain high-speed datatransmission where skew is unlikely to occur.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a diagram showing a configuration of a data transmissiondevice 1 a according to Example 1 of the present invention.

FIG. 2 is a diagram showing transition of the potential of atransmission line path 30 shown in FIG. 1.

FIG. 3 is a diagram showing a change in the output impedance of a driver10 and the impedance of a variable impedance element 40 over time.

FIG. 4A is a diagram showing a configuration of a data transmissiondevice 1 b according to Example 1 of the present invention.

FIG. 4B is a diagram showing a configuration of a data transmissiondevice 1 a according to Example 1 of the present invention.

FIG. 5A is a diagram showing a configuration of a variable impedanceelement 42 shown in FIG. 4A.

FIG. 5B is a diagram showing a configuration of a variable impedanceelement 44 shown in FIG. 4B.

FIG. 6 is a diagram showing a configuration of an output buffer 12 a ofthe driver 10.

FIG. 7A is a diagram showing a configuration of a variable impedanceelement 46.

FIG. 7B is a diagram showing a configuration of a variable impedanceelement 48.

FIG. 8A is a diagram showing a configuration of a data transmissiondevice 2 a according to Example 2 of the present invention.

FIG. 8B is a diagram showing impedance characteristics of diodes 181 to184.

FIG. 9 is a diagram showing a configuration of a data transmissiondevice 2 b according to Example 2 of the present invention.

FIG. 10 is a diagram showing a configuration of a data transmissiondevice according to another example of the present invention.

FIG. 11 is a diagram showing a configuration of a conventional datatransmission device 200.

FIG. 12 is a diagram showing transition of the potential of atransmission line path 230 shown in FIG. 11

BEST MODE FOR CARRYING OUT THE INVENTION

Hereinafter, examples of the present invention will be described withreference to the accompanying drawings.

EXAMPLE 1

FIG. 1 shows a configuration of a data transmission device 1 a accordingto Example 1 of the present invention. The data transmission device 1 aincludes a driver 10 for sending data, a receiver 20 for receiving thedata sent from the driver 10, and a transmission line path 30 forconnecting between the driver 10 and the receiver 20. The data istransmitted from the driver 10 to the receiver 20 via the transmissionline path 30. Each of the driver 10 and the receiver 20 is, for example,a semiconductor integrated circuit.

The data transmission device 1 a further includes a variable impedanceelement 40 the impedance value of which varies automatically accordingto the potential of the transmission line path 30. One end of thevariable impedance element 40 is connected to an end on the receiver 20side of the transmission line path 30. The other end of the variableimpedance element 40 is connected to a terminal potential V_(term).

The driver 10 includes an output buffer 12 for outputting data onto thetransmission line path 30. The output buffer 12 is connected via a pad14 to the transmission line path 30.

In the example shown in FIG. 1, the output buffer 12 is of a push-pulltype. The output buffer 12 includes a PMOS transistor 71 p and an NMOStransistor 71 n. The gates of the transistors 71 p and 71 n receivepredetermined logic values determined by a NAND element 73, a NORelement 74, and operational amplifiers 75 and 76. The operationalamplifier 75 receives the potential of the transmission line path 30 anda reference potential VR₁. The operational amplifier 76 receives thepotential of the transmission line path 30 and a reference potentialVR₂.

In an initial state, the transistor 71 p is in the OFF state, and thetransistor 71 n is in the OFF state. In this initial state, when dataData having a value ‘1’ is input into the output buffer 12, thetransistor 71 p is switched ON. The transistor 71 n remains in the OFFstate. As a result, the potential of the transmission line path 30 isincreased to be close to a predetermined potential V_(CCQ). Thereafter,when the potential of the transmission line path 30 becomes more thanthe reference voltage VR₁, the transistor 71 p is switched OFF. Thetransistor 71 n remains in the OFF state. This is because when thepotential of the transmission line path 30 becomes more than thereference potential VR₁, the output of the operational amplifier 75 goesto the LOW level and, as a result, the gate of the transistor 71 p goesto the HIGH level.

In an initial state, the transistor 71 p is in the OFF state, and thetransistor 71 n is in the OFF state. In this initial state, when dataData having a value ‘0’ is input into the output buffer 12, thetransistor 71 n is switched ON. The transistor 71 p remains in the OFFstate. As a result, the potential of the transmission line path 30 isdecreased to be close to a predetermined potential V_(SSQ). Thereafter,when the potential of the transmission line path 30 becomes less thanthe reference voltage VR₂, the transistor 71 n is switched OFF. Thetransistor 71 p remains in the OFF state. This is because when thepotential of the transmission line path 30 becomes less than thereference potential VR₂, the output of the operational amplifier 76 goesto the HIGH level and, as a result, the gate of the transistor 71 n goesto the LOW level.

As described above, the output buffer 12 of the driver 10 switches OFFthe transistor 71 p when the potential of the transmission line path 30becomes greater than the reference potential VR₁, and switches OFF thetransistor 71 n when the potential of the transmission line path 30becomes less than the reference potential VR₂.

The receiver 20 includes an input buffer 22 for receiving data from thetransmission line path 30. The input buffer 22 is, for example, anoperational amplifier having two input terminals.

One input terminal of the input buffer 22 is connected via a pad 24, astub resistor 32, and a resistor 31 to the transmission line path 30.The other input terminal of the input buffer 22 is connected to theterminal potential V_(term). The terminal potential V_(term) is, forexample, 1.1 V.

The input buffer 22 determines whether data on the transmission linepath 30 has the HIGH level or the LOW level using the terminal potentialV_(term) as a reference potential. Thus, the input buffer 22 receivesthe data sent from the output buffer 12.

Note that a node which has the same potential as that of the terminalpotential V_(term) may be provided separately from the terminalpotential V_(term). In this case, using the potential of this node as areference potential, the input buffer 22 can determine whether data onthe transmission line path 30 has the HIGH level or the LOW level.Therefore, the input buffer 22 is unaffected by the noise of theterminal potential V_(term).

A variable impedance element 40 includes a diode 81 and a diode 82 whichare connected to each other in parallel. The direction (forwarddirection) of a current flowing through the diode 81 is opposite to thedirection (forward direction) of a current flowing through the diode 82.

When the potential of the transmission line path 30 is around theterminal potential V_(term), the diodes 81 and 82 are not biased in theforward direction. Therefore, the potential of the transmission linepath 30 is around the terminal potential V_(term), and the impedancevalue of the variable impedance element 40 is much increased.

When the output buffer 12 outputs HIGH-level data onto the transmissionline path 30 so that the potential of the transmission line path 30 isincreased to (V_(term)+V_(f)), the diode 82 is biased in the forwarddirection. As a result, the impedance value of the variable impedanceelement 40 is much decreased. Here V_(f) denotes a forward voltage ofthe diode 81 or 82.

When the output buffer 12 outputs LOW-level data onto the transmissionline path 30 so that the potential of the transmission line path 30 isdecreased to (V_(term)−V_(f)), the diode 81 is biased in the forwarddirection. As a result, the impedance value of the variable impedanceelement 40 is much decreased.

FIG. 2 shows transition of the potential of the transmission line path30 when HIGH-level data and LOW-level data are alternately output fromthe driver 10.

When data transmitted from the driver 10 is in the transition state, thepotential of the transmission line path 30 transits from the HIGH levelto the LOW level (or the LOW level to the HIGH level) at a constant highspeed. This is because when the potential of the transmission line path30 is around the terminal potential V_(term), the impedance value of thevariable impedance element 40 has a large value so that only a loadcorresponding to the capacitance of the transmission line path 30 isapplied to the output buffer 12 of the driver 10.

On the other hand, when the data transition is completed to some degreeso that the potential difference between the potential of thetransmission line path 30 and the potential of the terminal voltageV_(term) becomes large, the impedance value of the variable impedanceelement 40 is decreased. This is because the potential of thetransmission line path 30 is increased to (V_(term)+V_(f)) so that thediode 82 of the variable impedance element 40 is biased in the forwarddirection; and the potential of the transmission line path 30 isdecreased to (V_(term)−V_(f)) so that the diode 81 of the impedanceelement 40 is biased in the forward direction. For this reason, an upperlimit of the amplitude of data transmitted from the driver 10 is clampedto the potential (V_(term)+V_(f)) and a lower limit of the amplitude ofthe data is clamped to the potential (V_(term)−V_(f)). As describedabove, the amplitude of the data transmitted from the driver 10 islimited to a predetermined range (V_(term)−V_(f) to V_(term)+V_(f)). Asa result, it is possible to transmit data having small amplitude.

For example, when the diodes 81 and 82 are Schottky diodes, the forwardvoltage V_(f) is about 0.4 V. Therefore, the potential of data on thetransmission line path 30 swings between 1.5 V and 0.7 V where theterminal potential V_(term) of 1.1 V is the middle value.

When the data transition is completed, the potential difference betweenthe potential of the transmission line path 30 and the terminalpotential V_(term) as a reference potential is substantially equal tothe forward voltage V_(f) of the diodes 81 and 82 of the variableimpedance element 40 regardless of the output impedance of the driver10. This can provide a sufficient potential difference between thepotential of the transmission line path 30 and the terminal potentialV_(term). As a result, the logical determination can be securelyperformed.

Note that a resistor 31 connected in series between the variableimpedance element 40 and the transmission line path 30 is used in orderto restrict a current flowing between the terminal potential V_(term)and the driver 10 when the diodes 81 and 82 are biased in the forwarddirection.

Further, when the reference potentials VR₁ and VR₂ of the output buffer12 of the driver 10 are set to around the potentials (V_(term)+V_(f))and (V_(term)−V_(f)), respectively, a direct current flowing between theterminal potential V_(term) and the driver 10 can be removed. This isbecause when the potential of the transmission line path 30 is thepotential (V_(term)+V_(f)) or the potential (V_(term)−V_(f)), thetransistors 71 p and 71 n of the output buffer 12 are switched OFF sothat the output impedance of the driver 10 becomes very large. In thiscase, the potential of the transmission line path 30 maintains thepotential (V_(term)+V_(f)) or the potential (V_(term)−V_(f)) due to thecapacitance of the diodes 81 and 82 and the capacitance of thetransmission line path 30 itself. Therefore, the potential differencerequired for the logical determination in the receiver 20 issubsequently held.

FIG. 3 shows variations in the output impedance of the driver 10 and theimpedance value of the variable impedance element 40 over time. In anexample shown in FIG. 3, it is assumed that the output impedance of thedriver 10 and the impedance value of the variable impedance element 40each have one of two values. In FIG. 3, the highest of the two values isrepresented by ‘H’ and the lowest is represented by ‘L’.

When data on the transmission line path 30 does not transit, both theoutput impedance of the driver 10 and the impedance value of thevariable impedance element 40 are set to ‘H’ (time period T₁). For thisreason, a direct current flowing between the driver 10 and the variableimpedance element 40 can be removed.

When data on the transmission line path 30 transits from the LOW levelto the HIGH level, the output impedance of the driver 10 is set to ‘L’(time period T₂). For this reason, the potential of the transmissionline path 30 transits at a high speed.

Thereafter, the potential of the transmission line path 30 is increasedto the potential (V_(term)+V_(f)) or is decreased to the potential(V_(term)−V_(f)), and the impedance value of the variable impedanceelement 40 is set to ‘L’ (time period T₃). For this reason, thetransmission line path 30 is terminated so that the transmitted data isnot reflected and has small amplitude.

Thereafter, when the potential of the transmission line path 30 becomesgreater than the reference potential VR₁, or when the potential of thetransmission line path 30 becomes less than the reference potential VR₂,the output impedance of the driver 10 is set to ‘H’ (time period T₄).This is because when the potential of the transmission line path 30becomes greater than the reference potential VR₁, or when the potentialof the transmission line path 30 becomes less than the referencepotential VR₂, the transistors 71 p and 71 n of the output buffer 12both are switched OFF. For this reason, the potential of thetransmission line path 30 transits toward the terminal potentialV_(term), so that the potential of the transmission line path 30 becomesless than the potential (V_(term)+V_(f)) or greater than the potential(V_(term)−V_(f)). As a result, the impedance value of the variableimpedance element 40 is set to ‘H’ (time period T₅).

In the time period T₅, the output impedance of the driver 10 and theimpedance value of the variable impedance element 40 both are set to‘H’. For this reason, a direct current flowing between the driver 10 andthe variable impedance element 40 can be removed.

Note that when the reference potential VR₁ is set to be equal to thepotential (V_(term)+V_(f)) and the reference potential VR₂ is set to beequal to the potential (V_(term)−V_(f)), the output impedance of thedriver 10 changes from ‘L’ to ‘H’ while the impedance value of thevariable impedance element 40 changes from ‘H’ to ‘L’.

The same applies to the case where data on the transmission line path 30transits from the HIGH level to the LOW level (time periods T₆ to T₉).

As described above, the impedance value of the variable impedanceelement 40 and the output impedance of the driver 10 vary in associationwith each other.

According to the data transmission device 1 a, a direct current flowingbetween the driver 10 and the variable impedance element 40 can beremoved. Even when such a direct current is removed, the logic level ofdata on the transmission line path 30 can be held. This plays a role ina reduction in power consumption in a time period of no data transition.

For example, a probability of data transition is about 10% in the CPU ofa computer. Therefore, the effect of the low power consumption is moresignificant in a time period of no data transition than in a time periodof data transition.

For example, data having an amplitude of 1 V is transmitted at afrequency of 500 MHz using the conventional data transmission device 200shown in FIG. 11. In this case, a current consumed by the conventionaldata transmission device 200 is as follows. Note that it is assumed thatthe capacitance of the transmission line path 230 is 20 pF, and a directcurrent flowing through the terminator resistor 240 is 8 mA.

i) alternating current: 1 V×20 pF×500 MHz×10% (transition probability)=1mA

ii) direct current: 8 mA×90% (non-transition probability)=7.2 mA

As described above, a direct current component is predominantly consumedin the fast-speed data transmission where the amplitude of data islimited. Therefore, the removal of this direct current component largelycontributes to a reduction in power consumption.

FIG. 4A shows a configuration of a data transmission device 1 baccording Example 1 of the present invention.

The data transmission device 1 b includes a variable impedance element42 having a variable impedance controlled according to a control signal.One terminal 42 a of the variable impedance element 42 is connected toan end on a receiver 20 side of a transmission line path 30. The otherterminal 42 b of the variable impedance element 42 is connected to aterminator potential V_(term).

The impedance value of the variable impedance element 42 is changedaccording to control signals CTL₁ and CTL₂ input from the outside of thevariable impedance element 42. The control signal CTL₁ is input to thevariable impedance element 42 from a driver 10. The control signal CTL₂is input to the variable impedance element 42 from a receiver 20.

The driver 10 includes an output buffer (DB) 12 for outputting data ontothe transmission line path 30. The receiver 20 includes an input buffer(RB) 22 for receiving data from the transmission line path 30.

The output buffer 12 controls the variable impedance element 42 so thatthe fast-speed data transmission and the low power consumption areoptimized. For example, before outputting data onto the transmissionline path 30, the output buffer 12 controls the variable impedanceelement 42 in such a manner that the impedance value of the variableimpedance element 42 is decreased. For example, the impedance value ofthe variable impedance element 42 is controlled in such a manner as tobe in agreement with the characteristic impedance of the transmissionline path 30. These controls are carried out using the control signalCTL₁. This makes it possible to transmit data at a high speed.Thereafter, when the data transmission is completed, the output buffer12 controls the variable impedance element 42 so as to increase theimpedance value of the variable impedance element 42. This prevents adirect current from flowing between the variable impedance element 42and the driver 10. As a result, power consumption by the datatransmission device 1 b is decreased.

Note that the output buffer 12 is preferably controlled in such a mannerthat when the impedance value of the variable impedance element 42 ishigh, the output impedance of the driver 10 is high; and when theimpedance value of the variable impedance element 42 is low, the outputimpedance of the driver 10 is low.

Alternatively, instead of using the output buffer 12, the input buffer22 may control the impedance value of the variable impedance element 42.For example, when the input buffer 22 is in a standby state where thebuffer 22 can receive data from the transmission line path 30, the inputbuffer 22 controls the variable impedance element 42 in such a manner asto decrease the impedance value of the variable impedance element 42.Such a control is carried out using the control signal CTL₂. Thereafter,when the data transmission is completed, the input buffer 22 controlsthe variable impedance element 42 in such a manner as to increase theimpedance value of the variable impedance element 42. This prevents adirect current from flowing between the variable impedance element 42and the driver 10. As a result, power consumption by the datatransmission device 1 b is decreased.

Note that the output buffer 12 is preferably controlled in such a mannerthat when the impedance value of the variable impedance element 42 ishigh, the output impedance of the driver 10 is high; and when theimpedance value of the variable impedance element 42 is low, the outputimpedance of the driver 10 is low. Such a control is, for example,carried out by supplying a control signal CTL₃ into the output buffer 12from the input buffer 22.

As described above, in the data transmission device 1 b, the impedancevalue of the variable impedance element 42 and the output impedance ofthe driver 10 are controlled depending on whether data is beingtransmitted or not. Alternatively, the impedance value of the variableimpedance element 42 and the output impedance of the driver 10 may becontrolled in a way as shown in FIG. 3. In the control shown in FIG. 3,the state where data is being transmitted is divided into sub states sothat the impedance value of the variable impedance element 42 and theoutput impedance of the driver 10 are more suitably controlled duringtransmission of data.

FIG. 5A shows a configuration of a variable impedance element 42. Thevariable impedance element 42 includes resistors R₁ to R₄ which areconnected in series to each other between a terminal 42 a and a terminal42 b and switches SW₁ to SW₄ and SW′₁ to SW′₄ which are provided forbypass, corresponding to R₁ to R₄, respectively.

The ON-OFF for the switches SW₁ to SW₄ is controlled with the controlsignal CTL₁. The ON-OFF for the switches SW′₁ to SW′₄ is controlled withthe control signal CTL₂. When the switches SW′₁ to SW′₄ are all in theOFF state, the impedance value of the variable impedance element 42 canbe changed in four levels by switching ON or OFF the switches SW₁ to SW₄according to the control signal CTL₁. When the switches SW₁ to SW₄ areall in the OFF state, the impedance value of the variable impedanceelement 42 can be changed in four levels by switching ON or OFF theswitches SW′₁ to SW′₄ according to the control signal CTL₂.

FIG. 4B shows a data transmission device 1 c according to Example 1 ofthe present invention. The data transmission device 1 c includes acontroller 50 for controlling a variable impedance element 44 in such amanner that the impedance value of the variable impedance element 44 canbe changed.

A CPU 60 provides the controller 50 with information indicating anoperating speed of the CPU 60. The information indicating an operatingspeed of the CPU 60 is, for example, information indicating an operatingmode of the CPU 60 (e.g., a normal operating mode, alow-power-consumption operating mode, and the like). Alternatively, theinformation indicating an operating speed of the CPU 60 may beinformation indicating an operating clock frequency.

The controller 50 determines based on the information provided by theCPU 60 whether the CPU 60 is operated at a high speed or not.

When the CPU 60 is operated at a high speed, the controller 50 controlsthe variable impedance element 44 in such a manner as to decrease theimpedance value of the variable impedance element 44. Such a control ofthe variable impedance element 44 is carried out using a control signalCTL₅. The decreased impedance of the variable impedance element 44allows high-speed data transmission.

On the other hand, when the CPU 60 is operated at a low speed, thecontroller 50 controls the variable impedance element 44 in such amanner as to increase the impedance value of the variable impedanceelement 44. Such a control of the variable impedance element 44 iscarried out using the control signal CTL₅. The increased impedance ofthe variable impedance element 44 prevents a direct current from flowingbetween the variable impedance element 44 and the driver 10. As aresult, power consumption by the data transmission device 1 c isreduced.

Thus, both high-speed data transmission and low power consumption can beachieved at a system level by adjusting the impedance value of thevariable impedance element 44 according to the operating speed of theCPU 60.

Further, when the CPU 60 is operated at a high speed, the controller 50preferably controls the output buffer 12 in such a manner that theoutput impedance of the driver 10 is decreased. Such a control of theoutput buffer 12 is carried out using the control signal CTL₄. Thedecreased output impedance of the driver 10 allows high-speed datatransmission. When the CPU 60 is operated at a low speed, the controller50 preferably controls the output buffer 12 in such a manner that theoutput impedance of the driver 10 is increased. Such a control of theoutput buffer 12 is carried out using the control signal CTL₄. Theincreased output impedance of the driver 10 prevents a direct currentfrom flowing between the variable impedance element 44 and the driver10. As a result, power consumption by the data transmission device 1 cis reduced.

FIG. 5B shows a configuration of a variable impedance element 44. Thevariable impedance element 44 includes resistors R₁ to R₄ which areconnected in series to each other between a terminal 44 a and a terminal44 b and switches SW₁ to SW₄ which are provided for bypass,corresponding to R₁ to R₄, respectively.

The ON-OFF for the switches SW₁ to SW₄ is controlled with the controlsignal CTL₅. The impedance value of the variable impedance element 44can be changed in four levels by switching ON or OFF the switches SW₁ toSW₄ according to the control signal CTL₅.

FIG. 6 shows a configuration of an output buffer 12 a of the driver 10.The output buffer 12 (FIG. 1) can be replaced with the output buffer 12a.

The output buffer 12 a includes a push-pull transistor for outputtingdata onto the transmission line path 30. The push-pull transistorincludes two sets of transistors having different sizes. Specifically,the output buffer 12 a includes a set of a PMOS transistor 91 p and anNMOS transistor 91 n having large sizes, and a set of a PMOS transistor92 p and an NMOS transistor 92 n having small sizes.

The gates of the transistors 91 p and 91 n receive predetermined logicvalues determined by a NAND element 73, a NOR element 74, andoperational amplifiers 75 and 76. The operational amplifier 75 receivesthe potential of the transmission line path 30 and a reference potentialVR₁. The operational amplifier 76 receives the potential of thetransmission line path 30 and a reference potential VR₂.

The gates of the transistors 92 p and 92 n receives the output of aninverter 78. The inverter 78 receives data Data.

In transition of data on the transmission line path 30, the outputbuffer 12 a switches ON either of the transistors 91 p and 92 p or thetransistors 91 n and 92 n according to the value of data to betransmitted. This allows the potential of the transmission line path 30to change at a high speed.

When the potential of the transmission line path 30 becomes more thanthe reference potential VR₁, the transistor 91 p is switched OFF. Thetransistor 92 p remains ON. When the potential of the transmission linepath 30 becomes less than the reference potential VR₂, the transistor 91n is switched OFF. The transistor 92 n remains ON.

Such a control allows a micro amount of direct current to flow throughthe transmission line path 30 via the transistors 92 p and 92 n duringno transition of data.

The transistors 92 p and 92 n and the diodes 81 and 82 actively maintainthe potential of the transmission line path 30 at the potential(V_(term)+V_(f)) or (V_(term)−V_(f)). As a result, an improvedcharacteristic is obtained where data is lesser influenced by noise.

FIG. 7A shows a configuration of a variable impedance element 46. FIG.7B shows a configuration of a variable impedance element 48. Thevariable impedance element 44 (FIG. 1) can be replaced with the variableimpedance element 46 or 48.

The variable impedance element 46 includes a resistor 93 connected inseries to the diodes 81 and 82 connected in parallel. One end of theresistor 93 is connected to the terminal potential V_(term). The otherend of the resistor 93 is connected via the diodes 81 and 82 to thetransmission line path 30.

The variable impedance element 48 includes a resistor 94 connected inseries to the diodes 81 and 82 connected in parallel. One end of theresistor 94 is connected via the diodes 81 and 82 to the terminalpotential V_(term). The other end of the resistor 94 is connected to thetransmission line path 30.

The variable impedance elements 46 and 48 have extremely high impedancesbefore one of the diodes 81 and 82 is biased in the forward direction.When one of the diodes 81 and 82 is biased in the forward direction, thevariable impedance element 46 has an impedance substantially equal tothe impedance of the resistor 93 and the variable impedance element 48has an impedance substantially equal to the impedance of the resistor94.

Thus, the impedances of the variable impedance elements 46 and 48 afterthe diodes 81 or 82 have been biased in the forward direction becomeshigher as compared with the impedance value of the variable impedanceelement 44 (FIG. 1). Therefore, it is possible to reduce the peak valueof a current into the driver 10 when the diode 81 or 82 is biased in theforward direction.

Further, the resistors 93 and 94 each preferably have a resistance equalto the characteristic impedance Z of the transmission line path 30. Thisprevents reflection from occurring at an end on the receiver 20 side ofthe transmission line path 30.

Further, the forward voltage V_(f) of the diodes 81 and 82 issubstantially in agreement with an amplitude of the potential of thetransmission line path 30 from the terminal potential V_(term), theamplitude being generated when the driver 10 outputs HIGH-level data,and with an amplitude of the potential of the transmission line path 30from the terminal potential V_(term), the amplitude being generated whenthe driver 10 outputs LOW-level data.

Assume, for example, that the impedance of the transmission line path 30and the impedances of the resistors 93 and 94 both are 50 ohm, theterminal potential V_(term) is 1.1 V, and the output impedance of thedriver 10 is 50 ohm. In this case, when the driver 10 outputs HIGH-leveldata, the potential of the transmission line path 30 is 1.65 V. When thedriver 10 outputs LOW-level data, the potential of the transmission linepath 30 is 0.55 V. Since the amplitude of data from the terminalpotential V_(term) is 0.55 V, the forward direction voltage V_(f) of thediodes 81 and 82 is preferably set to 0.55 V.

EXAMPLE 2

FIG. 8A shows a configuration of a data transmission device 2 aaccording to Example 2 of the present invention. The data transmissiondevice 2 a performs data transmission in a so-called differential mode.

The data transmission device 2 a includes a driver 110 for sending data,a receiver 120 for receiving the data sent from the driver 110, andtransmission line paths 130 and 131 connecting between the driver 110and the receiver 120. Positive-logic data is transmitted from the driver110 to the receiver 120 via the transmission line path 130.Negative-logic data is transmitted from the driver 110 to the receiver120 via the transmission line path 131.

The data transmission device 2 a further includes a variable impedanceelement 140 the impedance of which is automatically changed according tothe potential of the transmission line path 130, and a variableimpedance element 141 the impedance of which is automatically changedaccording to the potential of the transmission line path 131. Thevariable impedance element 140 is connected to an end on the receiver120 side of the transmission line path 130. The variable impedanceelement 141 is connected to an end on the receiver 120 side of thetransmission line path 131.

The variable impedance element 140 includes diodes 181 and 182. Theanode of the diode 181 is connected via the resistor 191 to the terminalpotential V_(term1). The cathode of the diode 181 is connected to thetransmission line path 130. The anode of the diode 182 is connected tothe transmission line path 130. The cathode of the diode 182 isconnected via the resistor 192 to ground V_(SS).

Note that the resistors 191 and 192 can be omitted. When the resistor191 is omitted, the anode of the diode 181 is connected to the terminalpotential V_(term1). When the resistor 192 is omitted, the cathode ofthe diode 182 is connected to ground V_(SS).

The variable impedance element 141 includes diodes 183 and 184. Theanode of the diode 183 is connected via the resistor 193 to the terminalpotential V_(term2). The cathode of the diode 183 is connected to thetransmission line path 131. The anode of the diode 184 is connected tothe transmission line path 131. The cathode of the diode 184 isconnected via the resistor 194 to ground V_(SS).

Note that the resistors 193 and 194 can be omitted. When the resistor193 is omitted, the anode of the diode 183 is connected to the terminalpotential V_(term2). When the resistor 194 is omitted, the cathode ofthe diode 184 is connected to ground V_(SS).

The driver 110 includes an output buffer (DBT) 112 for outputting dataonto the transmission line path 130 and an output buffer (DBC) 113 foroutputting data onto the transmission line path 131. The output buffer112 is connected via a pad 114 to the transmission line path 130. Theoutput buffer 113 is connected via a pad 115 to the transmission linepath 131.

The receiver 120 includes an input buffer 122 for receiving data fromthe transmission line paths 130 and 131. The input buffer 122 is, forexample, an operational amplifier having two inputs.

One of the inputs of the input buffer 122 is connected via a pad 124 anda stub resistor 132 to the transmission line path 130. The other of theinputs of the input buffer 122 is connected via a pad 125 and a stubresistor 133 to the transmission line path 131.

The variable impedance element 140 is designed to satisfy a conditionsuch that the sum of the forward direction voltages V_(f) of the diodes181 and 182 is greater than the potential difference between theterminal potential V_(term1) and the ground V_(SS). The variableimpedance element 141 is designed to satisfy a condition such that thesum of the forward direction voltages V_(f) of the diodes 183 and 184 isgreater than the potential difference between the terminal potentialV_(term2) and the ground V_(SS). For example, the above-describedconditions are satisfied when the terminal potentials V_(term1) andV_(term2) each are 1.5 V, and the forward direction voltages V_(f) ofthe diodes 181 to 184 each are 1.0 V.

The satisfaction of the above-described conditions prevents a directcurrent from flowing through the terminal potentials V_(term1) andV_(term2) to the ground V_(SS) when the outputs of the drivers 110, andthe transmission line paths 130 and 131 are floating.

FIG. 8B shows the impedance characteristics of the diodes 181 to 184. Inan example shown in FIG. 8B, it is assumed V_(DD)=V_(term1)=V_(term2).Alternatively, the potential V_(term1) may differ from the potentialV_(term2).

When the potential of the transmission line path 130 is between thepotential (V_(SS)+V_(f)) and the potential (V_(term1)−V_(f)), thecharacteristics of both diodes 181 and 182 connected to the transmissionline path 130 are both in a high impedance region (see FIG. 8B).Therefore, in this case, the variable impedance element 140 has anextremely high impedance. As a result, data on the transmission linepath 130 transits at a constant high speed.

When the potential of the transmission line path 130 is higher than thepotential (V_(SS)+V_(f)) , the characteristic of the diode 182 is in alow impedance region (see FIG. 8B). When the potential of thetransmission line path 130 is lower than the potential (V_(term1)−V_(f)), the characteristic of the diode 181 is in a low impedance region (seeFIG. 8B).

As described above, when the potential of the transmission line path 130is higher than the potential (V_(SS)+V_(f)), or when the potential ofthe transmission line path 130 is lower than the potential(V_(term1)−V_(f)), the characteristic of either the diode 181 or 182 isin a low impedance region. Therefore, in this case, the variableimpedance element 140 has an extremely low impedance around the terminalpotential V_(term1) or the ground V_(SS). This is because the diode 181or 182 is biased in the forward direction.

As a result, a potential (Hi-potential) indicating that data on thetransmission line path 130 is at the HIGH level is clamped around thepotential (V_(SS)+V_(f)). A potential (Lo-potential) indicating thatdata on the transmission line path 130 is at the LOW level is clampedaround the potential (V_(term1)−V_(f)). This restricts the amplitude ofdata.

For example, when (V_(SS)+V_(f))=1.0 V and (V_(term1)−V_(f))=0.5 V, adata amplitude is 0.5 V. Thus, data having such a small amplitude of 0.5V can be transmitted.

Note that the Hi-potential and Lo-potential of the transmission linepath 130 are determined by the resistors 191 and 192 and the outputimpedance of the output buffer 112. For example, the Hi-potential andLo-potential of the transmission line path 130 can be set to 1.0 V and0.5 V, respectively, by adjusting the output impedance of the outputbuffer 112.

Thus, the impedance value of the variable impedance element 140 ischanged according to the potential of the transmission line path 130.Similarly, the impedance value of the variable impedance element 141 ischanged according to the potential of the transmission line path 131.

Note that in order to prevent data reflection, the resistances of theresistors 191 to 194 are preferably equal to the characteristicimpedances of the transmission line paths 130 and 131.

Further, by increasing the output impedance of the output buffer 112after the potential of the transmission line path 130 becomes greaterthan the potential (V_(SS)+V_(f)) or less than the potential(V_(term1)−V_(f)), a direct current consumed by the driver 110 may besignificantly removed.

Similarly, by increasing the output impedance of the output buffer 113after the potential of the transmission line path 131 becomes greaterthan the potential (V_(SS)+V_(f)) or less than the potential(V_(term2)−V_(f)), a direct current consumed by the driver 110 may besignificantly removed.

FIG. 9 shows a configuration of a data transmission device 2 b accordingto Example 2 of the present invention. The data transmission device 2 bperforms data transmission in a so-called differential mode.

The data transmission device 2 b includes a variable impedance element142. An end 142 a of the variable impedance element 142 is connected tothe transmission line path 130. The other end 142 b of the variableimpedance element 142 is connected to the transmission line path 131.

The variable impedance element 142 includes diodes 185 and 186 connectedin parallel and a resistor 195. The configuration of the variableimpedance element 142 is similar to that of the variable impedanceelement 46 shown in FIG. 7A. The variable impedance element 142 can bereplaced with the variable impedance element 40 (FIG. 1) or the variableimpedance element 48 (FIG. 7B).

In the data transmission device 2 b, output buffers 112 and 113 canmonitor both the potentials of the transmission line paths 130 and 131.The output impedances of the output buffers 112 and 113 are set to highvalues after the potential difference between the potentials of thetransmission line paths 130 and 131 becomes greater than the forwardvoltage V_(f) of the diodes 185 and 186. Therefore, a direct currentconsumed by the driver 110 is significantly removed.

In Examples 1 and 2, it is described that data is transmitted from onedriver to one receiver (so-called point-to-point data transmission. Thisinvention is not limited to the point-to-point data transmission). Forexample, this invention can be applied to the case as shown in FIG. 10where data is transmitted from one driver to a plurality of receiversvia a transmission line path. In this case, the above-described variableimpedance element is provided at an end of the transmission line path.

INDUSTRIAL APPLICABILITY

As described above, a data transmission device according to the presentinvention can prevent a direct current from flowing through atransmission line path, thereby reducing power consumption. The datatransmission device of the present invention can prevent occurrence ofskew when data is latched using a clock signal, resulting in high-speeddata transmission.

What is claimed is:
 1. A data transmission device comprising: a driverfor sending data; a receiver for receiving data sent from the driver; atransmission line path for connecting between the driver and thereceiver; and a variable impedance element having a controllablyvariable impedance, wherein: the variable impedance element is connectedto the transmission line path; and the variable impedance element iscontrolled to have an impedance substantially equal to a characteristicimpedance of the transmission line path when the voltage of thetransmission line path is outside a predetermined range.
 2. A datatransmission device according to claim 1, wherein the impedance value ofthe variable impedance element is changed according to a potential ofthe transmission line path.
 3. A data transmission device according toclaim 1, wherein the impedance value of the variable impedance elementis changed according to a control signal input from the outside of thevariable impedance element.
 4. A data transmission device according toclaim 1, wherein the impedance value of the variable impedance elementand an output impedance of the driver are changed in association witheach other.
 5. A data transmission device according to claim 4, whereinthe output impedance of the driver is changed according to the impedancevalue of the variable impedance element.
 6. A data transmission deviceaccording to claim 1, wherein the variable impedance element includes afirst diode and a second diode connected in parallel, a direction of acurrent flowing through the first diode is opposite to a direction of acurrent flowing through the second diode.
 7. A data transmission deviceaccording to claim 6, wherein the variable impedance element furtherincludes a resistor connected in series to the first and second diodesconnected in parallel.
 8. A data transmission device according to claim7, wherein a resistance of the resistor is substantially equal to acharacteristic impedance of the transmission line path; and a forwarddirection voltage of the first and second diodes is substantially equalto an amplitude of a potential of the transmission line path from apredetermined terminal voltage, the amplitude being generated when thedriver outputs the data onto the transmission line path.
 9. A datatransmission device comprising: a driver for sending data; a receiverfor receiving data sent from the driver; first and second transmissionline paths for connecting between the driver and the receiver; a firstvariable impedance element having a first controllably variableimpedance; a second variable impedance element having a secondcontrollably variable impedance, wherein: the first variable impedanceelement is connected to the first transmission line path, and the secondvariable impedance element is connected to the second transmission linepath; the first variable impedance element is controlled to have animpedance substantially equal to a characteristic impedance of the firsttransmission line path when the voltage of the first transmission linepath is outside a first predetermined range; and the second variableimpedance element is controlled to have an impedance substantially equalto a characteristic impedance of the second transmission line path whenthe voltage of the first transmission line path is outside a secondpredetermined range.
 10. A data transmission device according to claim9, wherein the first variable impedance element includes first andsecond diodes; the anode of the first diode is connected to apredetermined first potential; the cathode of the first diode isconnected to the first transmission line path; the anode of the seconddiode is connected to the first transmission line path; and the cathodeof the second diode is connected to a predetermined second potentiallower than the predetermined first potential; the sum of the forwarddirection voltages of the first and second diodes is greater than apotential difference between the predetermined first potential and thepredetermined second potential; the second variable impedance elementincludes third and fourth diodes; the anode of the third diode isconnected to a predetermined third potential; the cathode of the thirddiode is connected to the second transmission line path; the anode ofthe fourth diode is connected to the second transmission line path; andthe cathode of the fourth diode is connected to a predetermined fourthpotential lower than the predetermined third potential; and the sum ofthe forward direction voltages of the third and fourth diodes is greaterthan a potential difference between the predetermined third potentialand the predetermined fourth potential.
 11. A data transmission deviceaccording to claim 9, wherein: the driver includes a first buffer foroutputting data onto the first transmission line path and a secondbuffer for outputting data onto the second transmission line path; theimpedance value of the first variable impedance element and an outputimpedance of the first buffer are changed in association with eachother; and the impedance value of the second variable impedance elementand an output impedance of the second buffer are changed in associationwith each other.
 12. A data transmission device comprising: a driver forsending data; a receiver for receiving data sent from the driver; atransmission line path for connecting between the driver and thereceiver; and a variable impedance element having a controllablyvariable impedance, wherein: the variable impedance element is connectedto the transmission line path; the variable impedance element includes afirst diode and a second diode connected in parallel, a direction of acurrent flowing through the first diode is opposite to a direction of acurrent flowing though the second diode; the variable impedance elementfurther includes a resistor connected in series to the first and seconddiodes connected in parallel; a resistance of the resistor issubstantially equal to a characteristic impedance of the transmissionline path; and a forward direction voltage of the first and seconddiodes is substantially equal to an amplitude of a potential of thetransmission line path from a predetermined terminal voltage, theamplitude being generated when the driver outputs the data onto thetransmission line path.
 13. A data transmission device comprising: adriver for sending data; a receiver for receiving data sent from thedriver; first and second transmission line paths for connecting betweenthe driver and the receiver; a first variable impedance element having afirst controllably variable impedance; and a second variable impedanceelement having a second controllably variable impedance, wherein: thefirst variable impedance element is connected to the first transmissionline path, and the second variable impedance element is connected to thesecond transmission line path; the first variable impedance elementincludes first and second diodes wherein the anode of the first diode isconnected to a predetermined first potential, the cathode of the firstdiode is connected to the first transmission line path, the anode of thesecond diode is connected to the first transmission line path, and thecathode of the second diode is connected to a predetermined secondpotential lower than the predetermined first potential; the sum of theforward direction voltages of the first and second diodes is greaterthan a potential difference between the predetermined first potentialand the predetermined second potential; the second variable impedanceelement includes third and fourth diodes: the anode of the third diodeis connected to a predetermined third potential; the cathode of thethird diode is connected to the second transmission line path; the anodeof the fourth diode is connected to the second transmission line path;and the cathode of the fourth diode is connected to a predeterminedfourth potential lower than the predetermined third potential; and thesum of the forward direction voltages of the third and fourth voltagesis greater than a potential difference between the predetermined thirdpotential and the predetermined fourth potential.